1. FIELD OF THE INVENTION
The present invention relates to an apparatus for measuring viscoelasticity and, in particular to such an apparatus capable of tracing the change in viscoelasticity of vulcanized rubbers and thermosetting resins as they are cured.
2. DESCRIPTION OF THE PRIOR ART
Viscoelasticity testing is carried out to detect whether or not vulcanized rubbers and thermosetting resins have the designed physical properties after unvulcanized stock is subjected to vulcanizing processes or thermosetting resins are subjected to thermosetting processes. As for a rubber, ISO-3417-1977 "Rubber-Measurement of Vulcanization Characteristics with the Oscillating Disc Curemeter" and The Japan Rubber Manufacturers Association Standard SRIS 3105-1980 "Measurement of Curing Characteristics with Oscillating Type Curemeter" prescribe methods for testing viscoelasticity.
FIG. 1 is a view showing a model example of a conventional apparatus for measuring viscoelasticity employing an SRIS 3105-1980 B-type die.
In FIG. 1, an upper die 1 (or the die for torque detection) as grooved in its working lower face a lattice pattern. An upper stationary die 3 (the stationary die on the torque detection side) is mounted enclosing the upper die 1 via an upper ring seal 5 (the seal on the torque detection side). The upper die 1, the upper stationary die 3 and the upper seal 5 constitute an upper die assembly B.
A lower die 2 (the driving die) also is provided in its upper working face with grooves of a lattice pattern and is surrounded by a lower stationary die 4 (the stationary die on the drive side) via a lower ring seal 6 (the seal on the drive side). The lower die 2, the lower stationary die 4 and the lower seal 6 constitute a lower die assembly A.
The lower die 2 is mounted on top of a drive shaft 8 in which a heating element 20 is embedded. The drive shaft 8 is connected to a drive mechanism which may consist of a motor 10, an eccentric motor drive shaft 11, and a crank arm 12. The upper die 1 is mounted on a torque detection shaft 7 in which a heating element 19 is embedded. The torque detection shaft 7 is provided with a torque arm 13 which is connected to a load cell 14.
The upper die assembly B includes a heating plate 15 having a heating element 21 to heat the heating plate and is supported by an upper base 17. Also, the lower die assembly A includes a heating plate 16 having a heating element 22 to heat the heating plate and is supported by a lower base 18. A pneumatic cylinder 23 is connected to the upper base 17 to pneumatically drive the latter in vertical direction.
The apparatus of FIG. 1 may be operated in the following steps for measurement of viscoelasticity.
(1) First, the pneumatic cylinder 23 is actuated to pull upward the upper base 17 to lift the upper die assembly B away from the lower die assembly A. Then, a specimen of unvulcanized stock, hereinafter referred to as simply "rubber specimen", is placed on the working surface of the lower die 2 of the die assembly A.
(2) The pneumatic cylinder 23 is restored to lower the upper base 17 until the upper die 1 is pressed against the lower die 2, with the rubber specimen being packed within a specimen chamber 26 which is the sealed cavity formed between the die assemblies A and B.
(3) While the heating elements 19, 20, 21 and 22 keep the rubber specimen at a predetermined temperature the lower die 2 is rotationally oscillated by the drive mechanism so that the oscillating force is applied to the rubber specimen. The torque mediated by the rubber specimen is transmitted to the upper die 1 and detected by load cell 14.
In this manner, the change in the curing degree of the rubber specimen is measured as a function of time. Extra portions of the rubber specimen are ejected out onto a flash channel 24.
FIG. 2 is an enlarged cross-sectional view of a die assembly which meets the prescriptions of SRIS 3105-1980. The die assembly is substantially similar in construction to FIG. 1, and like components are indicated by like reference numerals, except for numerals 27 and 28 designated means for measuring temperature, respectively.
In those prior apparatuses, to secure an exact torque transmission, in the working surfaces of the dies, and those of disc, which will later be described, grooves are cut to prevent slip of the rubber specimen. Despite these devices, those conventional apparatuses have suffered the disadvantage of failing in effective torque transmission.
FIGS. 3A and 3B are partial plan and cross-sectional views, respectively, of an upper or a lower die for showing grooves 31 which meet the prescriptions of SRIS 3105-1980 type B die. Each of the grooves 31 is cut to a width of 1 mm and a depth to 0.5 mm. Because of the lattice pattern (or check pattern) of the grooves 31 which are not cut in the same directions as the flow of filled rubber specimen, the grooves have tended to have air bubbles left retained. These air bubbles affect greatly the precise torque transmission.
Grooves of a die prescribed in ISO-3417-1977 are shown in FIG. 4, and those of a biconical disc prescribed in the same standard are shown in FIGS. A and 5B. The specimen is filled in a sealed cavity formed between the opposed working faces of the upper and lower dies, and in operation is subjected to oscillation by the disc. FIG. 4 is a plan view showing the under surface of the upper die 32. The grooves 33 as depicted in FIG. 4, are arranged in a radial pattern, equally angularly spaced at an angle of 20.degree., cut to a width of 1.6 mm and a depth of 0.8 mm. The working face of the lower die, not shown, may also be cut to a similar groove pattern and dimensions.
FIGS. 5A and 5B are a top and side view, respectively, of a biconical disc 34 which is inserted in the sealed cavity formed between the working faces of the upper and lower dies and it will oscillate the filled rubber specimen. As illustrated in FIGS. 5A and 5B, in the upper working face of the disc 34 alternating long grooves 35 and short grooves 36 are formed. These grooves are arranged in a radial pattern, equally angularly spaced at an angle of 10.degree.. The width and depth of each of grooves is both 0.8 mm, and the length of long and short grooves are 12.5 mm and 7.5 mm, respectively. Also, the lower working face of the disc 34 is provided with alternating long 9.5 mm and short 7.5 mm grooves, cut to the same width and depth as the upper surface.
SRIS 3105-1980 states the prescription as disc type viscoelasticity tester A similar to ISO-3417-1977 regarding a radial groove pattern. The radial groove pattern, unlike latticed groove pattern, has the advantage that the air bubbles which might possibly mingle with are likely to be released together with the flash, since the grooves are cut in the same directions as the rubber specimen would flow upon filling into the sealed cavity formed between the upper and lower dies. However, in the prior art techniques, the use of these radial grooves has not satisfactory effects in preventing slip, even with specimens relatively hard to slip. This will be discussed in more detail.
FIG. 6 is a partial perspective view of the die or disc provided with radial grooves as shown in FIGS. 4, 5A and 5B, cut along a cylindrical surface having its center at the axis of rotation of the die or disc. The section is shown with emphasizing shade. The character F designates the flat portions between adjacent grooves while G indicating the bottom of each groove. In addition, the character f represents the circumferential dimension of the flat portions F. The breadth of bottoms G is represented by the character g. It then follows that f+g is equivalent to the pitch of the grooves. The ratio .alpha. of g to groove pitch (f+g) is defined by the following equation (1): EQU .alpha.=g/(f+g) (1)
In an apparatus for measuring viscoelasticity, a pair of metal surfaces (for example, an upper and a lower die; or a disc and a die), each surface having grooves cut therein, are opposed at predetermined intervals to form a sealed cavity. The rubber specimen to be tested is filled into the sealed cavity. Then, one of the surfaces is put into rotational oscillation which exerts a shearing stress on the specimen. This shearing stress in the specimen in oscillation is collected as a torque into the central shaft of the detecting surface of the other metal surface. This torque is measured by a load cell or a torque meter which is to bear the rotary force of the shaft. This is the principle of measurement utilizing a torsional oscillating type viscoelasticity measurement apparatus. In this method, as shearing stress generated by the oscillating metal surface increases, the specimen increases its tendency of slipping at the interface with metal. The flat portions F offer no resistance against the slipping, while only the bottoms G in the grooved surfaces prevent the slippage as gripping the specimen. Since the shearing stress generated in the specimen is substantially equal on the flat portions F and on the bottoms G, the shearing stress for each pitch is proportional to (f+g). Also, the force to prevent slippage is proportional to g. Therefore, the ratio of g to (f+g), .alpha. defined by equation (1), correlates with the maximum degree for the linear grooves to grip the rubber specimen and prevent its slippage. Accordingly, this ratio a will hereinafter be referred to as "grip ratio".
Conventionally, the grooves having a uniform width, as illustrated in FIGS. 4, 5A and 5B have been employed, as a result the value of .alpha., while greater around the center of the dies or discs, tended to become smaller near their periphery. According to the theory of the linear elasticity, the relationship between torque M and effective radius R of a die or disc is: EQU M.alpha.R.sup.3 for the conical plate type (2) EQU M.alpha.R.sup.4 for the parallel disc type (3)
Equations (2) and (3) teach that, in either a die or a disc, the peripheral portions are essential for generating the torque. This means that, unless a proper enough magnitude of .alpha. is maintained over the entire surface down to the periphery, it would be difficult to obtain stable reading of true values of shearing stress without slippage disturbances. In conventional measuring devices, .alpha. has tended to be a minimum at the periphery which is the most important part, making it difficult altogether to prevent the slippage of specimen. As a concrete example, computation indicates that .alpha. is approximately 0.26 at the periphery for each of FIGS. 4, 5A and 5B.
In the prior art, as an experimental auxiliary method for preventing slippage, roughening the working surface of precision machined dies or discs by sand blasting and also making fine unevenness on the flat portions F shown in FIG. 6 is proposed and this method is considered to be effective in preventing the slippage. However, introduction of this method is to admit that the linear grooves, primarily intended for such prevention, fail to serve the very purpose. Furthermore, the roughening of the surface, while temporarily effective in the initial stages of operation, will lose effects in the passage of time since the metal surfaces are increasingly contaminated with specimen. To restore the surfaces, brushing is required, which will in turn wear out the fine unevenness of the surface. Here, it is difficult to maintain reliable measurements throughout time.
As stated above, in the conventional apparatus for measuring viscoelasticity, the slippage between the specimen and the dies or disc occurs easily and it is difficult to obtain accurate torque measurement. With thermosetting resin, in particular, errors of measurement have tended to become even greater because of slippage.